The present invention relates to a method of installing and connecting a subsea riser for an offshore operation and to a device designed to implement the method.
Known installation and connection methods are used for connecting subsea risers to seabed installations for example an oil production wellhead or a collector, which is connected to a set of wellheads, or else part of a fixed installation anchored to the seabed.
Two known types of method are used for connecting one end of a subsea riser to a seabed installation, a first type in which said end is guided and connected vertically to the installation, i.e. approximately perpendicular to the surface of the seabed, and a second type in which said end is connected horizontally to the seabed installation, approximately parallel to the surface of the seabed.
In the first type of method, the end of the subsea riser is made to approach the seabed installation vertically, by unreeling said riser from a surface ship. The reader may refer to document U.S. Pat. No. 4,457,378, which describes such a type of method. However, it is not very easy for the position of the end of the riser relative to the installation to be perfectly controlled when the surface ship is subject to the current or swell, and not only does the end of the riser, which is fragile, run the risk of deteriorating or being damaged by contact against the surface of the seabed or the seabed installation, but the seabed installation itself runs the risk of being damaged.
In contrast, methods of the second type avoid this hazard, since the subsea riser is firstly made in its entirety on the seabed and then the connection is made by means of subsea ROVs (remotely operated vehicles) which are put down on the seabed installation and pull the end of the subsea riser by means of a cable, or which are put down on said end and drive it right to the seabed installation. The other end of the subsea riser is then brought up to a surface installation, such as for example a platform. A method of this type is illustrated in document U.S. Pat. No. 5,975,803.
Thus, the end of the subsea riser is attached and connected to the seabed installation without striking it, thanks to the subsea robot that drives the riser in a continuous translational movement along the seabed. Moreover, this technique is particularly advantageous as it makes it possible to reduce the downtime of the surface vessel, which lays down the risers on the seabed. The vessel is then available for other operations, while the subsea robots proceed to connect the risers to installations.
However, in deep-sea production areas, the subsea risers are consequently very long, and they are ill-suited, on the one hand, for being driven along the seabed by subsea robots and, on the other hand, to be raised after one of their ends has been connected to the seabed installation.